iOS power states in simple English - ios

Question

As far as I know there are 3 states an app can be in.
a) Foreground
b) Background
c) Sleep (which is a state of the whole device)
The difference between the first two seems simple. But what exactly is the third state? And when does it happen? Is it when the device is locked?
And, quite importantly, does this happen after a long time, or does the device go to sleep if the user doesn't use it for a second, and reawakes the moment there's user action?
I've read different and seemingly contradictory pages about it, so either they were referring to different states or I just misunderstood what they were saying.
What am I trying to achieve: several things. But one of them is a timer that will execute an action every 10 seconds, and if the device was asleep so timers didn't run - I want it to run at the earliest possible time. Should I set a timer for every second and check if 10 seconds have passed (so that if a user checks the time on the phone for one second, the action will happen) or will it work automatically even if I set the timer for every 10 seconds?

Answer 1

The application lifecycle and the potential states your app can be in are more complex than you have stated.
Apple has comprehensive documentation on the application lifecycle and includes this diagram:
The main states you need to be aware of are:
Foreground Active - your app is actively executing in the foreground (ie it is the onscreen app)
Background - your app is actively executing in the background
Suspended - your app is in memory, but not currently executing.
It is important to note that your app only executes in the background for limited durations and under specific conditions.
It will execute in the background briefly as you transition from active to suspended in order to give you a chance to perform any required housekeeping prior to being suspended
It will execute in the background in response to specific background execution modes and permissions, such as location updates, Bluetooth or streaming audio.
When many people talk of their app "being in the background", what they really mean is that it is suspended. The Background state is an executing state while the Suspended state is a non-executing state. While suspended your app is still held in memory and will resume executing where it left off when it resumes an execution state.
Timers do not fire when your app is suspended. That is, expiration or firing of a timer will not bring your app from the suspended to background execution state.
Your best strategy for tracking missed events is to record when an event should fire (ie the fire Date) and check if that is the past when your app enters the Foreground Active state.
When your app is the foreground app and the device screen locks, timers will stop "ticking", so your strategy of setting a one second timer is probably the best approach for that scenario.
E.g. You have a 10 second timer, with 5 seconds elapsed when the device sleeps. After waking the device a further 5 seconds will elapse before the timer fires.
Note, however, there is a good chance that when the user does wake the device they will open some other app your app will be suspended.
You should also be aware that your app can be jettisoned (removed from memory) at any time when it is in the Suspended state if iOS needs memory. In this case when the user re-enters your app, it will restart from the top-left Unattached state and any in-memory variables will be lost.